Direct Cu-to-Cu interconnection is an important process for application in three-dimensional integrated circuit (3D IC) and wide band gap (WBG) devices in electronic packaging industry. Micro-bumping and transient liquid phase (TLP) bonding are relatively fast and cheap process in Cu-to-Cu interconnection. However, brittle intermetallic compounds (IMCs) usually form at bonding joints, resulting in poor performance on mechanical and electrical properties. Therefore, how to fabricate bonding joints with solid-solution interface instead of IMCs is worthy to investigate. Since Ga has low melting point and wide solubility in Cu and Ni fcc phase, we proposed a new process for fabricating solid-solution Cu-to-Cu joints using Ga-based pastes and Ni UBM. High-performance Cu-to-Cu joints on mechanical property, thermal stability, and electrical property are demonstrated. This approach gives a new idea for Cu-to-Cu interconnection, which can be a promising process for next-generation 3D IC and WBG devices packaging.